Which rules do you have to follow in order to implement an immutable class?

• All fields should be final and private.
• There should be not setter methods.
• The class itself should be declared final in order to prevent subclasses to violate the principle of immutability.
• If fields are not of a primitive type but a reference to another object:
  • There should not be a getter method that exposes the reference directly to the caller.
  • Don’t change the referenced objects (or at least changing these references is not visisble to clients of the object).